1. Field of the Invention
The present invention relates to a new and useful improvement in a process for the isomerization of monocyclic methyl-substituted aromatic hydrocarbon compounds of from 8 to 10 carbon atoms contained in a feedstock also containing ethylbenzene. The process comprises the use of a specific catalyst in a vapor phase reaction, said catalyst containing a crystalline aluminosilicate zeolite characterized by a constraint index, hereinafter defined, within the approximate range of 1 to 12. 2. Description of the Prior Art
The catalytic arrangement of alkyl groups present in alkyl aromatic hydrocarbons to provide one or more products suitable for use in the petroleum and chemical industries has heretofore been effected by a wide variety of catalysts. Acidic halides such as aluminum chloride, aluminum bromide, boron trifluoride-- hydrogen fluoride mixtures, etc. have been used in the rearrangement of alkyl benzenes to provide valuable intermediates which find utility in the synthesis of rubber, plastic, fibers and dyes. Other catalysts which have been used include solid siliceous cracking-type catalysts such as silica-alumina and clays and platinum deposited on silica-alumina. Although various catalysts possess one or more desired characteristics, a majority of catalysts heretofore employed suffer from several disadvantages. Acidic halides such as aluminum chloride, for example, are partially soluble in the feed material and are easily lost from the catalyst zone. Catalysts of this type are also uneconomical because of their extreme corrosiveness and requirement for recovery from the effluent products. Other catalysts of the heterogeneous type, such as silica-alumina, platinum on alumina, etc., do not possess sufficient acidity to provide effective conversion and necessitate the use of relatively high temperatures above the order of 427.degree. C. to 510.degree. C. through the entire isomerization process. Prolonged high temperatures of this order for these catalyst materials frequently lead to excessive coke formation which lowers the yield of desired product and necessitates frequent regeneration of the catalyst to remove coke. This results in reducing on-stream time and leads to high catalyst consumption due to loss of catalyst activity. Heterogeneous catalyst such as the crystalline aluminosilicates, both natural and synthetic, possess sufficient acidity but suffer the disadvantage of poor selectivity and aging as evidenced by "coke" make and the excessive amounts of disproportionated product formed in isomerization reactions.
A process in the art for isomerization of xylene is Octafining, extensively discussed in the literature as exemplified by:
1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770 (1955). PA0 2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955. PA0 3. Ciapetta, F. G., U.S. Pat. No. 2,550,531. PA0 4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189. PA0 5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 178.
The catalyst for use in such process is platinum on silicaalumina.
An improved catalyst for use in Octafining plants is taught by U.S. Pat. No. 3,856,872 to be of the ZSM-5 type of zeolite, whereby the process operates at high space velocities. Further, a process utilizing ZSM-5 type zeolites in acid form for vapor-phase conversion of a feedstock containing mixed C.sub.8 aromatics in the absence of added nitrogen is taught by U.S. Pat. No. 3,856,873. Even in such processes, especially when the catalyst has increased acid activity, there is a loss of xylene, the primary isomerization product, presumably due in part to disproportionation of xylenes and/or transalkylation of xylenes with ethylbenzene which may be present in the reaction system.
It is hereby proposed and demonstrated that by conducting a first step of the process at a temperature which provides from about 50 mole percent to about 75 mole percent conversion of ethylbenzene present in the reaction system for a period of time of from about 24 hours to about 96 hours, a second step of the process may be conducted at a temperature which provides only from about 15 mole percent to less than about 50 mole percent, preferably from about 15 mole percent to about 30 mole percent, conversion of said ethylbenzene with reduced occurrence of undesirable by-product reaction and with reduced xylene loss.